Fix ROC metric for CUDA tensors

[tridao]

What does this PR do?

Previously roc metric (and auroc) errors when passed in CUDA tensors,
due to torch.tensor construction without specifying device.
This fixes the error by using F.pad instead.

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[codecov]

Codecov Report

Merging #2304 into master will increase coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #2304   +/-   ##
======================================
  Coverage      88%     88%           
======================================
  Files          70      70           
  Lines        5501    5502    +1     
======================================
+ Hits         4834    4835    +1     
  Misses        667     667           

[justusschock]

@tridao thanks a lot for this fix. You are right, this didn't occur, since we don't have tests with GPU tensors.

[pep8speaks]

Hello @tridao! Thanks for updating this PR.

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2020-06-23 12:09:52 UTC

[justusschock]

@tridao I also added changes to the tests in your PR. I will resolve the pep8 issues, once these changes are passing :)

[williamFalcon]

please derive the device from the lightning module or lightning

[justusschock]

@williamFalcon this is just for testing the functional interface, where the computation is on the same device as the inputs :)

[mergify]

This pull request is now in conflict... :(

it is just for tests, for run on CPU and GPU